Orthogonal Frequency Division Multiplexing (OFDM) is a digital multi-carrier modulation scheme utilizing multiple closely-spaced, orthogonal sub-carriers. Each sub-carrier is modulated with a conventional modulation scheme (e.g., quadrature amplitude modulation) at a low symbol rate, maintaining data rates similar to conventional single-carrier modulation schemes in the same bandwidth. OFDM modulation provides economical, robust communications under poor channel conditions, such as narrowband interference and frequency-selective fading due to multipath propagation. The low symbol rate allows for the use of a guard interval between symbols, reducing inter-symbol interference. OFDM is deployed or planned for a variety of wireless litigation systems, including IEEE 802.16 (WiMAX), some IEEE 802.11a/g wireless LANs (Wi-Fi), IEEE 802.20 Mobile Broadband Wireless Access (MBWA), and the like.
One proposal for a new flexible wireless cellular communication system, which can be seen as an evolution of the 3G WCDMA standard, is 3G Long Term Evolution (3G LTE). This system will use OFDM as multiple access technique (called OFDMA) in the downlink and will be able to operate on bandwidths ranging from 1.25 MHz to 20 MHz. Furthermore, data rates up to, and even exceeding, 100 Mb/s will be supported for the largest bandwidth. For the uplink, a kind of pre-coded OFDM is employed, where the primary purpose of the pre-coding is to reduce the large peak-to-average (PAR) ratio commonly known to be one of the drawbacks with OFDM.
OFDM is uniquely suited for LTE for a number of reasons. Relatively low-complexity receivers, as compared to other access techniques, can be used in case of highly time-dispersive channels. Additionally, at least in theory, OFDM allows for very efficient usage of the available bandwidth. For example, in the case of only one user transmitting, it is possible to exploit the fact that the channel quality typically is very different at different frequencies (that is, the channel is said to be frequency selective). Also, since the information in OFDM is transmitted on a large number of sub-carriers, different modulation and coding can be applied on different sub-carriers, rather than using the same modulation and coding on all sub-carriers.
One of the main challenges of OFDM is to ensure that the sub-carriers are orthogonal to one another. This implies that, for example, frequency offset and phase noise must be maintained at a sufficiently low level. If the orthogonality is lost, information on one sub-carrier is leaked to other sub-carriers, primarily to the closest ones. This leakage is referred to as inter-carrier interference (ICI).
OFDMA allows several users to share the available bandwidth by allocating different sub-carriers to the different users, making the users orthogonal to one another. The allocation of sub-carriers may be dynamic, such as allocating a larger number of sub-carriers to users that have a larger amount of data to transmit. Unlike to the situation with a single user in OFDM, loss of orthogonality of the sub-carriers may be significant if the different users' signals are received with very different power, which may occur in the uplink or the downlink.
Two of the major factors giving rise to ICI are frequency error and Doppler spread. A frequency error is due to a mismatch between the transmitter and the receiver in generating the carrier frequency. A frequency error will also be manifest when the transmitter and the receiver would have identical frequency generators, but where one of the receiver or transmitter is moving relative to the other. For a multi-path channel, different paths will experience different Doppler frequency shifts, giving rise to a spread in the experienced Doppler frequency at the receiver side.
For OFDM, the ICI caused by a frequency error can be accurately modeled as:
            I      ⁡              (                  δ          ⁢                                          ⁢          f                )              =                            π          2                3            ⁢                        (                                    δ              ⁢                                                          ⁢              f                                      Δ              ⁢                                                          ⁢              f                                )                2              ,where δf is the frequency error and Δf is the carrier spacing between the sub-carriers. Since all the sub-carriers are affected by the same frequency offset, the frequency error may be removed prior to applying the FFT, to eliminate the ICI.
If instead the ICI is caused by Doppler spread, then if the paths are assumed to arrive from all directions with a uniform distribution (referred to as Jakes' model), the ICI can be accurately modeled as:
            I      ⁡              (                  f          D                )              =                            π          2                3            ⁢                        (                                                                                ⁢                              f                D                                                    Δ              ⁢                                                          ⁢              f                                )                2              ,where f D is the maximum Doppler frequency and Δf is the carrier spacing between the sub-carriers.
If the ICI caused by a frequency error or Doppler spread is assumed to have the same effect as additive white Gaussian noise (AWGN), then the total noise experienced by a receiver is simply calculated as N+I, where N is power of the AWGN and I is the ICI power. Consequently, the effective signal-to-noise ratio (SNR) experienced by the system can be expressed as
      SNR    eff    =            S              N        +        I              .  Using the effective SNR as defined above, it is easy to determine if ICI is an issue of not. It is also easily seen that the larger effective SNR that is required, the harder requirements there will be on keeping the ICI at a low level.
From these formulas, it is clear that a straightforward way to reduce the ICI is to increase the carrier spacing Δf . A known feature of OFDM is redundancy in the form of a cyclic prefix (CP) prepended to the useful part of each OFDM symbol of duration Tu. The minimum duration of the CP should be at least as long as the (expected) maximum delay spread of the channel where the system is supposed to operate. Since the carrier spacing is the reciprocal of Tu, increasing Δf means that Tu will be decreased, but the CP duration must be maintained. Accordingly, increasing Δf results in reduced spectrum efficiency.
Another strategy to reduce ICI is to estimate the ICI and then remove its impact on the received signal. In general, ICI cancellation is a complex operation that adds cost and increases power consumption in an OFDM receiver. There are two major reasons for the complexity of ICI cancellation. First, from a mathematical perspective, removing the impact of ICI involves computing the inverse to a very large matrix, which is a computationally intensive task. Second, to estimate the ICI, both the channel and the channel derivative must be estimated. Since ICI reduces the effective SNR, accurate channel estimation cannot be performed, resulting in poor estimates of the ICI. An iterative approach to ICI cancellation has been suggested in the art, beginning with initial channel estimation and ICI cancellation. Following the initial ICI cancellation, improved channel estimates are obtained from the signals from which the initial ICI estimate has been removed. An improved ICI estimate is then obtained using the improved channel estimates. This iterative procedure may be repeated to obtain the desired performance improvement. Such iterative ICI estimation is computationally complex, and introduces delay.
One known scheme for ICI cancellation relies on subtracting the ICI from different sub-carriers, rather than attempting to invert a matrix. While this approach yields a significant gain improvement, especially if used together with windowing, it has been shown that the gain remains far from that ideally possible if the ICI could be fully removed, primarily because the channel estimate, and in particular the channel change, are difficult to estimate with sufficient accuracy. ICI cancellation schemes known in the art are complex, and although some yield considerable improvement, in general the improvement is far below what is theoretically possible.
Prior art OFDM ICI cancellation has only been considered when all the sub-carriers are transmitted by the same user. That is, a signal is sent from one transmitter, over a plurality of sub-carriers, and is received by a single receiver.